Differential mechanisms commonly used on vehicles are of the sun and planet gear type and have a well known disadvantage that when one wheel is on a slippery surface such as mud or ice and the other wheel is on a firm surface capable of providing traction, the first wheel will simply spin as it receives all the available power transmitted to the differential.
Limited slip differential mechanisms have been proposed in an attempt to overcome this problem which restrict the extent to which one wheel can spin relative to the other but such differentials are more complex and, therefore, more costly to produce.
In European patent application EP-A-0326289 there is proposed an alternative differential mechanism comprising two output cam members rotatable about an axis, each said member having a single annular cam surface thereon of undulating form comprising pairs of mutually inclined surfaces, one of said cam members having a different number of said pairs of inclined surfaces thereon than the other of said cam members, the cam surfaces on the cam members being frusto conical and converging towards each other, and a plurality of cam followers having end surfaces engaging the cam surfaces of the output cam members, the arrangement being such that relative contra rotation of said output cam members causes the cam followers to slide axially, and an input element engaging the followers which are slidably supported by the input element and moving the followers circumferentially relative to the output cam members, the input element engaging with a radially outer portion of each cam follower to move the follower circumferentially relative to the cam output members, and supporting the two cam output members against axially outward movement during drive of the cam output members through the followers.
Such a differential will hereinafter be called a differential of the type referred.
In a preferred embodiment of the above differential the two cam output members engage the input element through respective roller element bearings.
A problem associated with this type of differential is that because of the different number of pairs of inclined surfaces on one cam surface as compared with the other cam surface, the differential gives different torque ratios for cornering left and right hand bends.
One method of relieving this problem would be to increase the number of cam followers which reduces the differences. However this solution would increase the tendency of the followers to tip, if the followers depth remains constant. This increases the stress and wear in the differential.
Another method of altering the torque ratio would be to reduce the cam angles. However this also increases axial loading on the followers raising stress and wear.
According to the invention there is provided a differential of the type referred, wherein at least one cam output member is in frictional engagement with the input element. Frictional engagement with one output member helps to equalise the torque ratio between left and right hand. If there is frictional engagement between both output members and the input element the torque ratio for the differential can be raised.
Preferably the frictional engagement between said one output member and input element is different to the frictional engagement between the other cam output member and the input element.
Preferably one cam output member frictionally engages the input element through a thrust washer which is rotationally fast with one of the input element and said one cam output member, and frictionally engages the other thereof.
Conveniently one of said cam output members has n pairs of surfaces thereon and the other of said cam output members has n-1 pairs of surfaces thereon, wherein the one cam output member having n pairs of surfaces thereon frictionally engages the input element through a higher friction loss device than said other output member.
Such a solution to the problem is particularly useful for rear wheel drive differentials which usually require a higher torque ratio than front wheel drive differentials.
Also according to the invention there is provided a differential of the kind referred, wherein at least one of said cam output members is caused to frictionally engage the input element through a friction washer, the frictional engagement being selected to provide a desired torque ratio.